Line finder synchronizer



March 1, 1949. D. H. RANSOM 2,462,896

LINE FINDER SYNCHRONIZER Filed Nov. 14, 1945 7 Sheets-Sheet 1 FIG. I.

COMMON EQUIPMENT TO OTHER LINKS DIAL PULSE 24: 1

SECOND LINK CIRCUIT LIN E RE S RCUIT RCUIT 'garras/ne;

IT- i March 1, 1949. D, H, RANSOM 2,499,999

LINE FINDER SYNCHRONI ZER Filed Nov. 14, 1945 7 sheets-sheet 2 MASTEROSCILLATO BUSY PULSE I SHAFER FREQUENCY DIVIDER IO KC BY y March 1,i949. D. H. RANSOM LINE FINDER SYNCHRONIZER 7 Sheets-Sheet 5 Filed Nov.14, 1945 March l, 17949. Q H, RANSOM 2,462,896

LINE FNDER SYNCHRONIZER Filed Nov. 14, 1945 7 sheets-:sheet 5 i IN VENTOR.

LAST REGISTER 1 15V ATTORNEY March l, 1949.

Filed NOV. 14, 1945 D. H. RANSOM LINE FINDER SYNGHRONIZER /37 /36 V34L38 3M I 146 l l I f I I I lI I 45 I "LE I 1 I 3A l PULSE -r- L I- I lsIIAPING I AMPLIFIER I I i I PuL sa I I Aim T I I IER I 47 IB- sYN.courrgql I| 333 l I I I j B+ I /a I I I GAIN I Pl I coN-rRoL I5I I I I IT Low PASS I I FILTER a I molo AMPLIFI Y I I58 I I I -E- I/MQI l Ll-II lI L f T I IHM-- l sI-:coNol B- COUNTER I FIG@ LAST GATEH:` 3B LAST COUNTI I I IN VEN TOR. D/I l//D H. PAA/60M March 1, 1949. v H RANSOM LINEFINDER SYNCHRONIZER 7 Sheets-Sheet '7 Filed Nov. 14, 1945 IN V EN TOR.

LAST LINK |NTERMED- IATE LINK Patented Mar. l, 19d-9 LINE FINDER SYNCHRONIZER Application November 14, 1945, Serial No. 628,610

(Cl. l79--18) 10 Claims. l

This invention relates to synchronizing circuits and more particularlyto the circuits especially useful in synchronizing line finders.

In a copending applic-irtion of E. M. Delcraine (5l), Serial No.628,613, filed Nov. 14, 1945, a telephone system is disclosed whereinthe tele-- phone lines are all brought into common electronicdistributor. Villen any line initiates a call, voltages are establishedon the corresponding distributor electrode which voltages aretransferred through to a plurality of link circuits each provided withline finders. One of the line finders operates to make connection withthe calling line and transmit the signals on to line selectors which inturn select the called line so that the communication is completed. Inthe system as disclosed in the above-identified copending application,the line finders are each provided with an oscillator which may operateat a frequency slightly lower than the rotary or repetition frequency ofthe distributor. The calling line provides series of pulses which, afterprocessing, may be of constant amplitude. These pulses together withpulses derived from the oscillating circuit are applied to a gate tubewhich will pass energy only When the pulse from the oscillator and theincoming calling pulses coincide. These passed pulses then serve tosynchronize the oscillator with the rotary distributor so that thischannel is continuously selected to the exclusion of any other callingline.

According to my invention, l provide means whereby the local osci atorin the line finder circuit operates at a multiple frequency of thedistributor rotation. Also, the distributor preferably is supplied withits driving voltage from a source which is a multiple frequency of thedistributor frequency and is frequency divided for the purpose ofsecuring more stable operation. The incoming signal then serves tosynchronize the local oscillator at the multiple fre-- quency instead ofat the actual distributor frequency. By this arrangement, an improvedsynchronization can be obtained since even if the ,local oscillator is apart of a cycle out of step with the multiple frequency source, it willonly throw off the timing of this synchronizing circuit by a smallfraction of the total distributor cycle. This local oscillator frequencyis divided so that it will produce control pulses for the line findergate slightly lower freq v icy than the distributor operation until suchas incoming signals are received and synchronizing is effected.

As an alternative feature, in accordance with my invention, the highfrequency source which serves to control thevdistributor may be useddirectly to synchronize the local oscillator upon receipt of a calledsignal. In this forrn, ythe incoming pulses serve to operate a secondgate circuit which in turn Will permit application of the masteroscillator voltage to the local-,os-

illa`tor of the link circuit for the purpose of synchronization. v

It is an objectJ of my invention to provide a multiple frequencyoscillator together with a frequency divider as a lovver frequencysourcaand means for synchronizing thernultiple' frequency oscillator tocontrol said lower frequency source.

It is anotherobject of my invention to `provide a high frequency mastersource together with a local source operating-at a slightly differentfrequency together with frequency dividers for energy from both of thesources andmeans responsive to signals for synchronizing the masteroscillator and the local oscillator to bring? the divided frequency ofthe two sources'into' Syn'- chronism. l

It is a further object of my invention to provide a synchronizing systemfor a line :under of the type in which the line nder is synchronizedwith the frequency of incoming pulses of a' channel by synchronizing' asourceof'energy at a substantially multiple` frequency of the pulserepetition frequency and dividing' vthis frequency for synchronizingpurposes.

It is a still further object of my invention', in the line finder ofthe-type generally set fortli in the objects as above, to provide ameans for syncl'ironizing the local oscillator with the' masteroscillator by means of energy 'directly therefrom, the application ofwhich is controlled in accordance with the received signals of saidchannel.

In a system incorporating the features of my invention, the signal orspeech currents' in the various lines or other channels` may be replacedat the exchange by a series of narrovv'pulses of amplitudecorrespondingto the amplitude of the original-current at thecorresponding tinie. rihe pulses are produced at sufficient rapidity sothat they denne substantially the signal envelope. In this manner byallotting'diferent timepositicns to each line, the signal or voicecurrents' Within the exchange may be distributed over acoinmon channeleach signal being repeated by a series of pulses displaced in time inaccordance with the distributor time position.` This distribution may bereadily accomplished by means of a cathode ray tube serving as adistributor which Will sequentially scan the lines connected to'predetermined terminals and respond if there is a signalling voltage onthe line. rIhe channels may be separated by time selection and may beapplied through time displacement means and a low-pass filter whichserves to reproduce the audio envelope to the same or anotherdistributor also coupled to the lines. The incoming signals may serve toadiust the time displacement means so that they will represent the timedifference between the time position of the calling line and theselected called line. The time displacement means may be an actual delayline of some form or an equivalent circuit which, while not producing anactual delay of the signals, will effectively serve to store the energyand release it after a predetermined interval equal to the desireddelay. In this manner, the interconnection of any one line with anyother line of the system may be accomplished. Upon making thisinterconnection, the communication signals may pass through the samedelay means between the interconnected lines. Furthermore, since thescanning cycle covers each of the lines connected to the distributor, asmany simultaneous connections may be made as there are time displacementtrunking channels within the exchange.

Preferably, means are provided responsive to the interconnection of thelines to tie up these lines so that they cannot be selected by anothersubscriber attempting to get the connection. If desired, anyconventional type of busy signal may be applied to the subscribers linewhen this condition exists so that he will know that he must Wait aninterval for the line to become free so that he can make the desiredconnection.

I provide synchronizing means for the line finder in which a frequencyhigher than the scanning frequency is generated and frequency divisionis provided to reduce it substantially to the scanning frequency. Thus,a more stable frequency with less phase displacement may be obtained aswell as other advantages which are outlined below.

While I have broadly outlined certain objects and features of myinvention, a better understanding of my invention and the objects andfeatures thereof may be had from the particular description of anembodiment and certain modications thereof made with reference to theaccompanying drawings, in which:

Fig. 1 is a block diagram illustrating the general circuit set up;

Figs. 2 and 3 are sectional circuit diagrams and views respectively, ofa distributor tube used in my system;

Figs. 4 to 8 inclusive, constitute a circuit diagram of a link exchangein accordance with my invention;

Fig. 4 illustrating the common equipment,

Fig. 5 showing the pulse forming equipment,

Fig. 6 the line finder equipment,

Fig. 7 the dial register equipment, and

Fig. 8 the line selecting equipment;

Fig. 9 is a diagram illustrating how Figs. 4 to 8 inclusive, should bearranged to illustrate the complete circuit; and

Fig. 10 is an alternative form of circuit including both a commonequipment and a line finder circuit which may be substituted as a wholefor Figs. 4 and 6 in accordance with my invention.

In an example of a telephone exchange incorporating my system asoutlined above, the system may be divided into three parts as shown inFig. 1: rst, all the subscribers lines, twenty for example, assignednumerals l to 2li, each of these lines having a subscriber subsetequipment such as 2l; second, the equipment common to al1 line circuits,hereafter referred to as common equipment 22; and third, a group of linkcircuits one of which is needed for each simultaneous call. Each of thelink circuits may be further subdivided into line finder circuit 23,dial pulse forming circuit 24, dial register circuit 25 and lineselecting circuit 2t. These several major components are interconnectedby wires 21-38 inclusive, as shown in Fig. 1. For the sake of simplicityin the description only one-way conversation is illustrated.

As shown, all lines l to 2B terminate in common equipment 22. Thisequipment 22 performs a scanning function, preferably by means of asuitable tube having an electronic beam which sweeps each of the linesin turn.

When one of these lines has a potential indicative of a callingcondition, the common equipment 22 applies signals over wires 21 and 28to all the link circuits in parallel and specifically to the line findercircuit 23 of the first link (chosen for discussion). This line finder23 operates to find the calling line and transfer the signals over wireE3 to the dial pulse forming circuit 24.

When dialing ensues, this circuit 24 produces dial pulses which arecounted and stored in dial register circuit 25. The dial pulse register25 then serves to control the line selector circuit 25 which maycomprise a delay line or other time displacement apparatus.

The incoming speech signals are then transferred from common equipment22 over wire 28, line iinder circuit 23, wire 33, line selector circuit210 and thence over wire 36 back to the common equipment 22, from whencethey are applied to the selected outgoing line, The part of Fig. 1comprising line lnder 23, dial pulse forming circuit 24, dial register25 and line selector circuit 2@ may be considered together as a linkcircuit. For certain embodiments of the system, a synchronizingfrequency may be fed from common equipment 22 over lead 29 to lineselector circuit 26 and line finder circuit 23 respectively. The

five leads 21, 28, 29, Se and 31 from common equipment 22 may also bemultiplied to other link circuits of the system as shown.

The distributor function of common equipment 22 may be performed by arotating distributor in the form of a cathode ray tube as illustrated indetail in Figs. 2 and 3. The distributor tube is indicated generally at39 and may comprise a cathode 4S, the usual grid 4I, focus and anodeelectrode 42, horizontal deflector plates 43 and vertical deflectorplates 44. Two-phase distributor currents from a suitable sweep controlmay be applied over leads 45, 4S, 41 and 48 to the horizontal andvertical delector plates respectively, so as to produce a cyclicrotation of the electron beam. At the target end of tube 39 are providedtwenty coupling targets 49 to 68 respectively, which are coupled withthe individual lines l to 2i) inclusive. These targets may comprisesecondary electron emissive elements associated with a common anode 69to provide dynodes all having a common output. A mask or screen 1t maybe provided, if desired, having apertures therein so that the electronbeam will impinge on each dynode only when the beam is aligned therewiththus preventing possible secondary emission from others. The output ofthe distributor tube 39 is connected from anode 69 over lead 1l, thensignal isolating circuits hereafter described to leads 21 and 28 whichgo to the line finder circuit as shown in Fig. 1. The

output from the line selecting circuit 26 may be applied as indicatedover line S5 to the grid 4| serving to modulate the beam in accordancewith the selected signal energy. Thus, referring to Fig. 1, the outputfrom lead 'H may be applied after suitable delay (produced in lineselecting equipment 25 as hereafter described) over lead 3d to grid lilto provide the desired communication channel between the chosen pair oflines.

The common equipment 22 is illustrated in Fig. 4. For illustrativepurposes a base frequency of 10,000 cycles per second has been selectedas the scanning rate of the rotating distributor. This frequency issufficiently high to reproduce voice frequencies with adequate fidelityfor transmission of speech. For the twenty-line system the basefrequency is derived from a 200 kilocycle stable oscillator 'i2preferably crystal controlled. This higher frequency is utilized sinceit is generally easier to build a more stable oscillator at the higherfrequencies than at the lower 10,000 cycle frequency which is to beused. Further'- more, in certain of the modications illustrated, the 200kilocycle wave may be utilized for other control purposes. Thesinusoidal frequency generated in master oscillator l2 is reduced to thebase frequency of ten kilocycles in frequency divider T3.

The output of frequency divider 'i3 is applied over 90 phase shifter 'i8to the vertical and horizontal sets of deiiecting plates i3 and i4 ofdistributor tube 39 herein diagrammatically illustrated. This will serveto rotate the beam at a frequency of 10,000 revolutions per second sothat each of the dynodes #39 to 58, illustrated in Figs. 2 and 3 and inthis figure, will be scanned once every 10,000ths of a second. Incominglines i, 5 and 29 are shown connected to the respective dynodes 4S, 53and 68.

At 2| is illustrated a typical subscriber subset (shown connected toline 5) for use in the system according to my invention. Such a subsetwill be connected to each of the incoming lines I to 29 inclusive. TheVoice transmitter 'l5 is connected in series with dial I5 and thenormally open switch hook '11. The receiver 'E8 is bridged permanentlyacross the line, since, for simplicity of illustration, no separateringing equipment has been illustrated. Accordingly, the signal forsummoning a called subscriber may be applied as a special tone whichwill be reproduced in receiver 'i8 to call the listener to the phone.

As in the usual equipment, switch hook ll is normally open. However,upon initiating a call, the switch becomes closed, completing a circuitin the calling line loop over low-pass filter i9 and the associatedlines at the sub-set, applying a negative potential from battery 80 tothe asso ciated dynode 53. Normally the dynode electrodes 49 to 88 areat the same potential as anode 69 so no current Hows. This negativepotential will produce a difference in potential and cause secondaryemission current to flow from the dynodes upon impingement of the beamof tube 39 thereon, producing a negative output pulse in output line ll.The pulses are preferably signal modulated to a depth of only 25 to 50per cent so that there will always be sufficient amplitude to furnishenergy to establish and maintain connections regardless of modulatingsignals. The negative pulses resulting from operation of the selecteddynode 53 are fed to the grid of inverter tube 8l. The anode circuit oftube 8l is coupled to the grid of clipper tube 82 which serves to clipthese pulses at a predetermined level to pass only the modulatedportions of the incoming pulses. Thus, the output of this tube,representing the speech signals, may be substantially 100 per centmodulated. These clipped pulses are then applied to a cathode followertube and from there to all of the link circuits over the cathodefollower output lead 28. A second output is taken across the cathoderesistance of inverter tube 8|, these pulses being applied to a clippertube it@ which serves to clip the pulses to a constant level eliminatingmodulation effects therefrom. The anode circuit of tube 84 is coupled tothe grid of a cathode follower tube 85 which serves to apply pulses 86through common feed resistor 8l' over wire 2l to the grid of signalfinder gate tube 88 (shown in Fig. 6) of line finder 23 (shown in Figs.6 and 1) in the rst link circuit (now under consideration) and inparallel to the grids of the corresponding line iinder gate tubes in allother links. The pulse St after passing through resistor 8l may becalled 89, so that the pulse actually arriving at the grid of tube 83and of the other similar tubes is pulse 09. Under the conditions nowassumed, when none of the grids of the line finder gate tubes is rawinggrid current, pulse 89 is nearly as strong as pulse 86; but under otherconditions it may be much weaker than 86. In the absence of any signalson the cathode of this signal finder gate tube 88, the above tracedpulse 89 on its grid is insufficient to cause the flow of plate current,because the bias applied to the grid is suiiciently far below cutoff.

A; line finder circuit incorporating my invention is illustrated in Fig.6. A lock-in oscillator 09 which incidentally performs a frequencydivision and is controlled through the medium of master oscillator l2 isprovided. The lock-in oscillator 90 operates at a frequency slightlyless than the two hundred kilocycles, its 50 kc. output being fedthrough a clipper differentiator circuit 0| to a kilocycle synchronizedmultivibrator 92. Before synchronization the 50 kc. and l0 kc. contentsalso will differ by corresponding intervals from these frequencies as inthe oscillator 90.

The output of multivibrator 92 is applied through the differentiatingnetwork 93 and 9G to tube 95 which serves to form and amplify the pulses96. Tube 95 is normally biased beyond cut-olf but the leading edge ofeach square wave output from multivibrator 92 is of sufficient strengthto first drive the grid positive on a portion of the square wave. Anegative pulse 96 of approximately ve microseconds is produced in theplate circuit. A cathode follower tube 9T passes the signal or controlpulse 8e to the cathode of line finder tube $8. When the signal 89 onthe grid of tube 88 coincides with the above-described selecting pulse98, the tube S3 conducts and passes a pulse 99 to four places, namely todiodes |0| and |82 and over wire 32 to the line selecting circuit and totube |09.

This pulse 99 is rectified in tube |02 and fed to an integrating network|03. The negative potential from the integrator is amplified in tube 0dreducing the potential in cathode resistor |05 which is common to tube|04 and tube |06. The reduction of this potential renders tube |84conductive. Thus, this tube IM now commences to pass the sine Wave frammaster oscillator 12, which is continuously applied to the grid thereofover line 29. This amplified wave is then passed through phase correctorcircuit |01 serving to lock-in oscillator 9|! with the master oscillator12. Accordingly, progression of selection is now stopped so that thepulses 89 will pass through tube 88 and over network Hi8 and tube H19 toopen line finder gate tube II@ at the correct instants, thus causing thelatter to pass the desired signal pulses from Wire 28 to Wire 33.

Simultaneously, the application of pulses 99 to vdiode lill charges.circuit lll and actuates tube I I2 which in turn actuates tube 95 toprevent enygagement of other line nders as is more fully Aset forth inthe aforementioned copending application.

A combination of a pulse register circuit and associated line selectioncircuit is shown in Figs. 7 and 8 respectively. Also, a form of pulseforining circuit is illustrated in Fig. 5. The circuits of Figs. 4 3arranged as shown in Fig. 9 illustrate a complete system incorporatingmy invention.

Turning first to Fig. 5, the incoming pulses I lilo on line 33 withtheir dialing breaks are passed through network U3, amplifier tube lll,transformer H5, tube lill, integrating netvvork ll'l and dial gate tubel it to an output line 35.

Dial gate tube il@ in Fig. 5 is normally biased to conduction by thevoltage on its suppressor from the dial gate control lli-3, l2@ (withtube Il@ normally conducting). Pulses passed by transformer H5 are alsoapplied over integrating network IEE, tubes |22 and |23, arranged in atime constant circuit, and over rectifier ld to the grid of tube i9. Thetime constant of the circuit is such that tube E23 remains operated fora period corresponding to the interval of the dial pulsing. Tube l lllcuts oi at the beginning of a series of dial pulses (sending out anineffective positive pulse) and reoperates at the end, sending out anegative pulse to the dial gate control lid, |20. This negative pulsefrom tube E23 cuts off tube I I9 causing tube l2@ to conduct. Thisbiases tube IIS to cut-oir" locking out the dial gate so that transientsand voice modulations or even additional dialing will not disturb theregisters (described later). At the same time, a control voltage is sentout over lead 38 biasing the control grid of the output gate tube |52(Fig. 8) for conduction.

Turning now to Figs. 7 and 8, the output line 135 is coupled to aplurality of trigger circuits IE5, |26, |21 and |23. As many of thesetrigger circuits are provided as there are subscribers lines. Normally,the right hand tubes of each pair (except ior the preliminary one |25)are cut-01T, biasing the respective gate tubes i253, i3d and i3d tocut-01T on their associated suppressor grids. When a series of dialpulses has been received, it is intended that one of the right handtubes of the trigger circuits will be left conducting, permitting itsgate tube to pass the signal.

The operation of the register may be described as follows: The firsttrigger circuit |25 has no associated gate and tube |32 is normallyconducting. A rst pulse over wire 35 from the pulse forming circuit cutsoff tube l32 of trigger circuit IZ causing tube '533 to conduct. Whentube |33 conducts, a negative pulse is sent from its plate circuit tothe grid oi the normally conducting tube I3?.` of trigger circuit lr6.cutting oli this tube and causing tube I 35 to conduct. This operationof tube |35 serves to open the rst gate tube |29` The second negativepulse from the pulse forming circuit cuts oir tube 235 transferringconduction to tube |34. A negative pulse from the plate of tube I 3dcuts off tube |35 restoring the rst gate to blocked condition whichtransfers conduction to tube H31 in the next ,register |21. The cut-01Tof tube |34 restores the 8 rst gate tube |29 to blocked conditionWhileth conduction of tube |31 unblocks the second gate tube |30. Thiscycle is repeated in succeeding registers until the last dial pulse hasbeen sent.

Thus, at the end of dialing only the gate corresponding to the number ofdial pulses is left unblocked. Assuming for convenience that line 3 iscalling line tl, the second gate i3d Will be unblocked. As a consequencethen, any pulses which may arrive on the grids of the gate tubes (e. g.over lines |38, H33, etc., described later) Will be passed only by gatei3@ to the common output Wire lflll as Will be described in more detailin connection With Fig. 8.

After the calling subscriber has hung up, the release circuit forrestoring the registers |25-|28 to normal Will be operated. This circuitis controlled so that tube lill is normally cut-off. When the line nder23 linds a line and locks-in, a signal is applied over line :il and tube42 so that the tube lill conducts. The resulting negative pulse fromtube Ml is passed on to tube |43 over condenser I lill but is noteiective since tube |43 is already cut oi. However, when the line finderreleases, tube lfll is returned to its normal cut-oli condition, passinga positive pulse to tube |43. ll'his is amplied as a negative pulse inthe plate circuit or" tube M3 serving to reset all of the registertrigger circuits lZii-ll to normal.

The manner in which the speech signals are delivered to the selectedcalled (e. g. line 5) will now be considered in detail in connectionWith Figs. 7 and 3. In this embodiment the speech signals from thecalling line are stored and then later released. To determine the properinstant for releasing these stored signals, the equipment counts thepulses which niark the time channels intervening between the calling andcalled line channels.

in order that this counting may start at the instant corresponding tothe calling lines time channel, a synchronizing pulse from the linefinder gate tube over line 32 is passed through the shaping amplier M5to the synchronizing counter Mld. Thereafter, pulses produced in shapingamplifier lli? under control of oscillator l2 carry forward theoperation of the system through counters such as Idd, Iii@ and l5@ in amanner similar to that described for the register circuits ltd- Mit Onlythree counters are shown corresponding to the three described gates |29,53|) and lill, and these counters and gates cooperate to control theinstant of release of speech energy to the called line. As the countersoperate successively, they apply potentials to Wires i33, |39, etc. Butif only gate i3d is open, only the potential on Wire le@ will pass tocommon Wire |451, thus properly timing the release of speech signals tothe called line as Will shortly appear.

The incoming energy from the line lnder selector gate tube 355 isapplied by means of line 33 to a low-pass filter and audio amplifier Mlwhich serves as a storage means for the incoming speech signals. Thesespeech signals from the output of .filter ibi are continuously appliedto the control grid of gate tube it. However, no energy can pass untilthe completion of dialing when the cut-off bias is removed from controlgrid of gate tube M52 by the gate control Iig, lit and further untilsuch time as the proper gate pulses are applied from Wire Edil tocut-ofi gate control tube |53 Which in turn removes the bias on theSuppresser grid of this tube i152. The latter bias will be removed atthe proper time relation depending upon which one of the gate tubes |29,|30 is open 9 at that time. The gate tube |52 will, therefore, be openedat the proper instant in accordance with the incoming line signalso thatthe energy from storage lter |51 will be passed only at the proper timefor application to the outgoing line over lead 36.

The foregoing description covers a complete system incorporating oneform of my invention. However, an alternative structure for use in thesystem using my invention is illustrated in Fig. 10, which shows analternative arrangement of line finder and common equipment. Accordingto this arrangement, the same master oscillator l2, frequency divider'I3 and phaser lll for controlling the sweep of the beam in tube 39 isused. A slightly modied form of coupling circuit for dividing the signaland synchronizing pulses is shown differing somewhat from thatillustrated in Fig. 4. The output negative pulse from distributor 39 isfed over line 'H to an inverter |5|l and then into two cathode followers|55, |55. The tube |56 passes the speech signal to line 28 extending toall the links. This signal has not had its modulation depth increasedsince this function is performed in the link circuits in this form. Thecontrol signal is clipped to constant amplitude in a slightly differentmanner with a clipping circuit comprising duo diode |58' which limitsthe amplitude of the signal to the grid of the cathode follower |55.This cathode follower feeds i through a series resistance 8l to thegrids of all the link circuit tubes 8S, as explained before.

These tubes are normally biased suilciently beyond cut-olf so ythatsignals SQ alone on the input electrode will produce no change in theoutput and as before, coincidence with signals derived from the localoscillator is necessary to produce any response. Instead of providing alock-in oscillator and clipper, I provide in this system a localoscillator |58 operating at two hundred hilocyeles -i-O-.1%. 89 from thetube |55 are applied to the grid of signal gate tube 88 while the outputfrom oscillator |58 is applied through two frequency dividingmultivibrators |59 and Hill to provide the desired pulses which operatethrough tubes 95 and 9T to apply a selecting signal 98 to the cathode of.this same tube Sli. The relationship between pulses 89 and 98 willprogress as previously described until such time as a selecting pulse9|! on the cathode of tube 8S is applied simultaneously with a controlpulse SS to the grid thereof. Thus, tube 88 passes a pulse S9 through tothe grid of tube l! of a delay flip-flop circuit comprising tubes ||5|and |52 thus triggering this flip-flop circuit to its abnormal conditionwith tube |62 conducting, sending to shaper |53 an abrupt voltage rise.This delay flip-nop circuit has a period of action adjusted by theconstants of the grid circuit of tube When it spontaneously returns tonormal, the voltage to the shaper |53 drops back abruptly thuscompleting a long positive pulse to the Shaper. The pulse shaper servesto differentiate this pulse and suppress the leading portion, theltrailing portion of which has a desired delay. This trailing portion isthen amplified and applied to oscillator |58 to synchronize it with themaster oscillator 72. The halting of the relative drift of these twooscillators stops the pulse progression of pulse 98 with respect to 39and serves to lock the line finder to the selected line as previouslydescribed. Upon locking into step, the pulses 9S from tube 88 arerectied in the gain of tube 95 and hence the amplitude of The outputpulses l@ the pulses Q5 and then llt which are applied to the cathode oftube d8. Because of the fact that a higher frequency is used for thelocal oscillator, a more stable operation and precise lock-in can beobtained.

The busy gate tubes it! and operate to impose upon the pulses 89 anupper limit somewhat lower than the limit imposed by clipper |51. Thisnew limit being high to hold a previously engaged line finder but lowenough to prevent engaging a new one. In performing this function, tubesltd and |65 act in a manner similar to duo diode clipper lill. At theinstant of arrival of a positive pulse from busy pulse shaper |66 uponthe grid of tube EE/i it becomes highly conductive and thus acts as adiode to prevent Wire 2l from rising above the potential of its cathode.Tube leb acts as a reverse clipper to discharge the negative potentialwhich would remain at the end of such pulses.

The modification shown in Fig. lo may be used in place of Figs. 4 and 6in combination with the circuit arrangement disclosed in Figs. 5, 7 and8. Furthermore, the synchronizing features shown in connection with linefinders in the two modiiications illustrated may be used in other typesof circuits than the complete telephone or exchange systems shownherein, as will be apparent to those skilled in the art. The use ofmultiple frequencies for this synchronizing purpose of the typedescribed in particular constitutes one of the principal features of myinvention. It should therefore be clearly understood that the specificdescription of the system and the modifications thereof given herein aremerely by Way of example and are not to be considered as limitations onthe scope of my invention as set forth in the objects thereof and in theaccompanying claims.

I claim:

1. A selector circuit for selecting a pulse train, the pulses of whichhave a predetermined repetition frequency, comprising a source of energyof a frequency slightly displaced from a multiple frequency of saidrepetition frequency, means for deriving from said energy selectorpulses having a repetition frequency slightly lower than said repetitionfrequency, receiver means for receiving said pulse train and saidselector pulses, means in said receiver means for selecting pulses .ofsaid pulse train when said pulses and said selector pulses coincidetherein, and means responsive to said selected pulses for tuning saidenergy source substantially to said multiple frequency whereby saidcircuit will continue to select said pulse train.

2. A method of selecting a pulse train, the pulses of which have apredetermined repetition frequency, comprising providing energy at afrequency slightly displaced from a multiple frequency of saidrepetition frequency, deriving from said energy selector pulses having arepetition frequency slightly lower than said repetition frequency.selecting pulses of said pulse train when said pulses and said selectorpulses coincide, tuning said energy source substantially to saidmultiple frequency in response to said selected pulses.

3. A selector system for selecting one channel of a plurality ofchannels of pulse trains of a predetermined pulse repetition frequency,comprising a local oscillator operating at a frequency slightly lowerthan a multiple frequency of said repetition frequency, frequencydividerv means for dividing the frequency from said -local oscillator toprovide control pulses at a repetition -Y rate slightly lower than saidpredetermined repetition rate, gate circuit means normally biased beyondcutoff so that said pulses normally will not pass, means for applyingsaid signal pulses and said control pulses to said gate circuit, meansto condition said circuit to pass said channel pulses upon simultaneousapplication of the pulses thereto, Aand means responsive to said pulsespassed by said gate circuit to bring said local oscillator to saidmultiple frequency whereby said signal pulses will be continuouslyselected.

4. In a selector system for selectingl one channel of a plurality ofchannels of pulse channels of a predetermined repetition frequency, themethod comprising generating oscillations at a frequency slightly lowerthan a multiple frequency of said repetition frequency, dividing thefrequency of said generated oscillations to provide control pulses at arepetition rate slightly lower than said predetermined repetition rate,mixing said signal pulses and said control pulses to pass said channelpulses upon simultaneous occurrence of said signal and control pulses,bringing said local oscillator t said multiple frequency in response tosaid passed pulses whereby said signal pulses will be continuouslyselected.

5. A selector system for selecting one channel wherein each of aplurality of `channels of pulse channels of a predetermined repetitionfre.- quency, comprising a master oscillator operating at a multiple ofsaid repetition frequency, a local oscillator operating at a frequencyslightly lower than said multiple frequency, frequency divider means fordividing the frequency from said local oscillator to provide controlpulses at a repou tition' rate slightly lower than said predeterminedrepetition rate, gate circuit means normally biased 'beyond cutoff` sothat said pulses normally will not pass, means for applying said signalpulses and said control pulses to said gate cir cuit to condition saidcircuit to pass said channel pulses upon simultaneous application of thepulses thereto, and means responsive to said pulses passed by said gatecircuit for passing energy from said master oscillator to said localoscillator to bring said local oscillator to said mul" tiple frequencywhereby said signal pulses will be continuously selected.

6. In a communication system of the type wherein each of a plurality ofchannels is represented by interleaved signal pulses of a predeterminedrepetition rate; means for assuring selection of one channel Vto theexclusion of others of said channels comprising a selector circuitcomprising a gate means normally conditioned so that said signal pulsesnormally will not pass, means for applying said signal pulses to saidgate circuit', a local oscillator operating at a frequency slightlylower than a predetermined multiple of Ithe repetition rate, frequencydivider means for dividing the frequency from said local oscillator toprovide control pulses at a repetition rate slightly low-er than saidpredetermined repetition rate, means for applying s-aid control pulsesto said gate means to condition said gate to select and passpulses, andmeans responsive to said pulses passed by said gate circuit to brings'aid local oscillator to said predetermined multiple of the repetition'rate whereby said signal pulses will be continuously selected.

7. In a telephone exchange system of the type wherein each 'of a.plurality of lines is coupled to separa-te distributor terminals andmeans is provided to scan said terminals successively and cyclically ata predetermined frequency to provide signal pulses of a predeterminedrepetition rate in response to the initiation of a call on a line, meansfor assuring selection of a calling line to the exclusion of others ofsaid lines comprising a source of energy at a multiple of saidpredetermined frequency, eans responsive to energy from said source toprovide said predetermined frequency for said scanning, and a linefinder circuit comprising a line finder gate tube, normally biasedbeyond cutoff so that said pulses normally will not pass, means forapplying said signal pulses to said gate tube, a local source of energynormally at a frequency slightly lower than the frequency of saidfirst-named source, means responsive to energy from said local source toprovide control pulses at a repetition rate slightly lower than saidpredetermined repetition rate, means for applying said control pulses tosaid line nder gate tube to lower the bias thereon so that said signalpulses will be selected and passed upon simultaneous application Aofsaid signal and control pulses thereto, and means responsive to saidpulses passed by said gate tube for applying energy from the first-namedsource to said local source to synchronize it with said first sourcewhereby said signal pulses will be continuously selected,

8. In a telephone exchange system of the type wherein each of aplurality of lines is coupled to separate distributor terminals andmeans is provided to scan said terminals successively and cyclically .ata predetermined f `equency to provide signal pulses of a predeterminedrepetition rate in response to the initiation of a call -on a line,means for assuring selection of a calling line to the exclusion ofothers of said lines comprising a line finder circuit comprising a linefinder gate tube normally biased beyond cutoff so that said pulsesnormally will not pass, means for applying said signal pulses to saidgate tube, a local oscile lator operating at a frequency slightly lowerthan a multiple of the frequency of said repetition rate, frequencydivider means for dividing the frequency from said local oscillator toprovide control pulses at a repetition rate slightly lower than saidpredetermined repetition rate, means for ap plying said control pulsesto said line lfinder gate tube to lower the bias thereon so that saidsignal pulses will be selected and passed upon simul taneous applicationof said signal and control pulses thereto, and means responsive to saidpulses passed by said gate tube to bring said local oscillator to themultiple frequency whereby said signal pulses will be continuouslyselected.

9. In a telephone exchange system of the type wherein each of aplurality of lines is coupled to separate distributor terminals andmeans is provided to scan said terminals successively and cyclically ata predetermined frequency to provide signal pulses of a predeterminedrepetition rate in response to the initiation of a call on a line, meansfor assuring selection of a calling line to the exclusion of others ofsaid lines comprising a master oscillator operating at a multiple ofsaid predetermined frequency, frequency divider means for providing saidpredetermined frequency for said scanning, a line finder circuitcomprising a line finder gate tube normally biased beyond cutoff so thatsaid pulses normally will not pass, means for applying said signalpulses to said gate tube, a local oscillator operating at a frequencyslightly lower than the frequency of said master oscillator, frequencydivider means for dividing the frequency from said local oscillator toprovide control pulses at a repetition rate slightly lower than saidpredetermined repetition rate, means for applying said control pulses tosaid line nder gate tube to lower the bias thereon so that said signalpulses will be selected and passed upon simultaneous application lofsaid signal and control pulses thereto, and means responsive to saidpulses passed by said gate tube to synchronize said local oscillatorwith said master oscillator whereby said signal pulses will becontinuously selected 10. In a telephone exchange system of the typewherein each of a plurality of lines is coupled to separate distributorterminals and means is provided to scan said terminals successively andcyclically at a predetermined frequency to provide signal pulses of apredetermined repetition rate in response to the initiation of a call ona line, means for assuring selection of a calling line to the exclusionof others of said lines -comprising a master oscillator operating at amultiple of said predetermined frequency, frequency divider means forproviding said predetermined frequency for said scanning, a line ndercircuit comprising a line finder gate tube normally biased beyond cutoffso that said pulses normally will not pass, means for applying saidsignal pulses to said gate tube, a local oscillator operating at afrequency slightly lower than the frequency of said master oscillator,frequency divider means for dividing the frequency from said localoscillator to provide control pulses at a repetition rate slightly lowerthan said predetermined repetition rate, means for applying said controlpulses to said line finder gate tube to lower the bias thereon so thatsaid signal pulses Will be selected and passed upon simultaneousapplication of said signal and control pulses thereto, and meansresponsive to said pulses passed by said gate tube for applying energyfrom said master oscillator to said local oscillator to synchronize itwith said master oscillator whereby said signal pulses will becontinuously selected.

DAVID H. RANSOM.

No references cited.

